Keep up to 4 in a 10-gallon aquarium. This gives them room to walk around and enjoy themselves when they are awake.
Substrate
Peat Moss is the best substrate for any millipede. They eat it and it provides them with many nutrients that they otherwise wouldn't get. It also gives them a place to burrow, which they enjoy.
Food
Millipedes LOVE cucumber, with fish food flakes sprinkled over the top. Many people will also put a piece of chalk in with their millipedes to give them calcium.
Humidity
Use a spray bottle to spray the inside walls, substrate, and hiding tube one to three times a day. Be sure never to spray water directly onto the millipede.
Accessories
Millipedes love to hide in paper towel tubes during the day. Be sure not to have anything in with your millipede that will harm it such as rocks, other insects, etc. If you bring in wood from outside, put it in the oven for 10-15 minutes at 375 F in order to kill any living things that may have been on it.